1. Field of the Invention
The embodiments discussed herein are related to a silicon carbide semiconductor device and a method of manufacturing a silicon carbide semiconductor device.
2. Description of the Related Art
In recent years, silicon carbide (SiC) has gained attention as a semiconductor to replace silicon (Si). The bandgap of silicon carbide, for example, is 3.25 eV for 4-layer periodic hexagonal silicon carbide (4H—SiC), about 3 times greater than that of silicon (=1.12 eV). Consequently, use of silicon carbide as a semiconductor enables upper limit operating temperatures to be increased. Further, for example, the breakdown field strength of silicon carbide is 3.0 MV/cm for 4H—SiC, about 10 times greater than that for silicon (=0.25 MV/cm). Thus, ON resistivity inversely proportional to the cube of the breakdown field strength is decreased and steady-state power loss can be reduced.
The thermal conductivity of silicon carbide, for example, is 4.9 W/cmK for 4H—SiC, about 3 times higher than that of silicon (=1.5 W/cmK). Thus, a semiconductor device that uses silicon carbide (hereinafter, silicon carbide semiconductor device) has a higher thermal cooling effect than a semiconductor device that uses silicon (hereinafter, silicon semiconductor device) and is advantageous in that a smaller cooling apparatus can be used. Also, since the saturation drift velocity of a silicon carbide semiconductor device is high at 2×107 cm/s, silicon carbide semiconductor devices are also excellent for high-speed operation. Thus, silicon carbide has superior property values as compared to silicon and therefore, application to power semiconductor devices (hereinafter, power devices), high-frequency devices, and devices operated at high temperatures is expected.
Currently, prototypes of metal oxide semiconductor field effect transistors (MOSFET), pn diodes, Schottky diodes, etc. as silicon carbide semiconductor devices are being developed and devices that surpass silicon semiconductor devices in terms of dielectric breakdown voltage and ON resistivity are appearing one after another. ON resistivity is a ratio of forward voltage to forward current during energization (=forward voltage/forward current). In the production (manufacture) of such a silicon carbide semiconductor device, control of the carrier density, conductivity type, etc. of a predetermined region inside the semiconductor substrate, which is formed from silicon carbide (hereinafter, SiC substrate), is necessary.
Thermal diffusion methods, ion implantation methods, etc. are commonly known methods for controlling the carrier density, conductivity type, etc. of a predetermine region inside a SiC substrate. Thermal diffusion methods are widely used to produce silicon semiconductor devices. However, the impurity diffusion coefficient inside silicon carbide is extremely small and therefore, use of a thermal diffusion method to control the conductivity type, carrier density, etc. of a predetermine region inside a SiC substrate is difficult. Therefore, in the production of a silicon carbide semiconductor device, in general, an ion implantation method is used. In the formation of an n-type semiconductor region, nitrogen (N), phosphorus (P), etc. is often used for implantation whereas aluminum (Al), boron (B), etc. is often used for implantation when a p-type semiconductor region is formed.
A large capacity/high breakdown voltage power device has a vertical device structure in which current flows in a vertical direction of the semiconductor device, i.e., from a front surface toward a back surface of the semiconductor device, and voltage is applied between the front surface and the back surface of the semiconductor device. Therefore, a large capacity/high breakdown voltage power device is configured to have an electrode on the front surface and the back surface, respectively. For example, in the case of a Schottky diode, a Schottky electrode is disposed in a substrate front surface side and in a substrate back surface, an ohmic electrode is disposed having an ohmic contact, which is a contact (electrical contact portion) with a silicon portion. Further, in the case of a MOSFET, a source electrode and a gate electrode are disposed in a substrate front surface, and a drain electrode, which is an ohmic electrode, is disposed in a substrate back surface.
Typically, in a conventional method of manufacturing a silicon carbide semiconductor device, first, a silicon carbide epitaxial layer is grown on a front surface of a SiC substrate and thereafter, an ohmic electrode is formed in a back surface of the SiC substrate by vapor depositing a metal and performing heat treatment. Further, in a silicon carbide semiconductor device equipped with an ohmic electrode, when the contact resistivity of the SiC substrate and the ohmic electrode is high, the current (ON current) flowing in the active region during device operation is low compared to a case where the contact resistivity is low and the same voltage is applied. Therefore, as a method of reducing the contact resistivity of the SiC substrate and the ohmic electrode, formation of an ion implanted layer of a high impurity concentration in the back surface of the SiC substrate by ion implantation has been proposed (for example, refer to Japanese Laid-Open Patent Publication Nos. 2003-086816 and 2006-324585).
Nonetheless, although the formation of a high-concentration impurity layer in the substrate back surface is effective in reducing the contact resistivity of the SiC substrate and the ohmic electrode as described above, through the earnest research of the inventors, the following problems were found to occur with the techniques described in Japanese Laid-Open Patent Publication Nos. 2003-086816 and 2006-324585. FIGS. 11, 12, and 13 are cross-sectional views depicting states of a conventional silicon carbide semiconductor device during manufacture. Typically, in a conventional method of manufacturing a silicon carbide semiconductor device, first, as depicted in FIG. 11, ion implantation 111 is performed in the back surface of a SiC substrate 101 and a high-concentration semiconductor having an impurity concentration that is higher than that of the SiC substrate 101 is formed in a surface layer of the back surface of the SiC substrate 101. Next, as depicted in FIG. 12, a nickel (Ni) layer 103 and a titanium (Ti) layer 104 are sequentially formed in the surface of the high-concentration semiconductor region 102, as a back surface electrode.
Next, an anneal process (heat treatment) is performed to sinter the nickel layer 103 and the titanium layer 104. Although the nickel layer 103 is converted to a silicide by this heat treatment and a nickel silicide layer 105 is formed as an ohmic electrode forming an ohmic contact with the SiC substrate 101 as depicted in FIG. 13, a layer of continuous deposited carbon (C) (hereinafter, deposited carbon layer) 106 is formed in the nickel silicide layer 105. Thus, it has been confirmed that under the conventional technique, the deposited carbon layer 106 is formed in the nickel silicide layer 105 and at the boundary of the deposited carbon layer 106, the back surface electrode (metal layer formed on the nickel silicide layer 105) peels, and a favorable back surface electrode having a low contact resistivity with the SiC substrate 101 cannot be formed.